Jacketed LED assemblies and light strings containing same

ABSTRACT

A jacketed light emitting diode assembly is provided, which includes a light emitting diode including a set of positive and negative contacts, and a lens body containing a semiconductor chip and end portions of the contacts. An electrical wire set of first and second electrical wires are connected to the positive contact and the negative contact, respectively. A light transmissive cover receives the lens body, and has an opening through which at least one of the contact set and the electrical wire set passes. An integrally molded plastic jacket at the opening of the light transmissive cover provides a seal at the opening against moisture and airborne contaminants. A waterproof light string including one or more of the jacketed light emitting diode assemblies is also provided, as are related methods.

This application is a Continuation-In-Part of U.S. patent applicationSer. No. 10/243,835 filed Sep. 16, 2002, now U.S. Pat. No. 6,830,358which is a Continuation of U.S. patent application Ser. No. 09/819,736filed Mar. 29, 2001, now U.S. Pat. No. 6,461,019.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to light emitting diode assemblies, lightstrings comprising a plurality of light emitting diode assemblies, andrelated methods.

2. Description of Related Art

Light emitting diodes (LEDs) are increasingly employed as a basiclighting source in a variety of forms, including decorative lighting,for reasons among the following. First, as part of an assembly, LEDshave a very long lifespan, compared with common incandescent andfluorescent sources. For example, a typical LED lifespan is at least100,000 hours. Second, LEDs have several favorable physical properties,including ruggedness, cool operation, and ability to operate under widetemperature variations. Third, LEDs are currently available in allprimary and several secondary colors, as well as in a “white” formemploying a blue source and phosphors. Fourth, with newer dopingtechniques, LEDs are becoming increasingly efficient, and colored LEDsources currently available may consume an order of magnitude less powerthan incandescent bulbs of equivalent light output. Moreover, withexpanding applications and resulting larger volume demand, as well aswith new manufacturing techniques, LEDs are increasingly cost effective.

Conventional LEDs are typically constructed using steel or coated stealcontacts or frames. LED contacts are also available in copper orcopper-alloys, although these materials generally are consideredundesirable because of their higher cost and incompatibility with someautomated LED manufacturing equipment and certain types of LED diematerial.

LED-containing holiday and decorative light sets, such as used fordecorative purposes such as for Christmas lighting, typically usecopper-alloy contacts to transfer electricity from the lead wires to theLED lamps. Although manufacturers take efforts to seal the contacts andconnections against moisture and airborne contaminants, it is difficultif not impossible to achieve completely and consistently a moisture andcontaminant seal.

LED-based light strings also present other drawbacks. For example, U.S.Pat. No. 5,495,147 entitled LED LIGHT STRING SYSTEM to Lanzisera(“Lanzisera”) and U.S. Pat. No. 4,984,999 entitled STRING OF LIGHTSSPECIFICATION to Leake (“Leake”) describe different forms of LED-basedlight strings. In both Lanzisera and Leake, exemplary light strings aredescribed employing purely parallel wiring of discrete LED lamps using astep-down transformer and rectifier power conversion scheme. These andall other LED light string descriptions found in the prior art convertinput electrical power, usually assumed to be the common U.S. householdpower of 110 VAC to a low voltage, nearly DC input.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an LED assembly capable ofaddressing one or more of the above-mentioned drawbacks.

It is another object of this invention to provide an LED assemblypossessing a complete and permanent barrier, especially for the metalcontacts and associated electrical connections, against moisture andcorrosive contaminants.

It is still another object of this invention to provide an LED assemblyhaving improved durability and longevity.

It is another object of the invention to provide a light stringcomprising a series of LED assemblies of the invention.

It is still another object of the invention to provide a method formanufacturing the assemblies and light-strings of this invention.

To achieve one or more of the foregoing objects, and in accordance withthe purposes of the invention as embodied and broadly described in thisdocument, according to a first aspect of this invention there isprovided a jacketed light emitting diode assembly, comprising a lightemitting diode, an electrical wire set, a light transmissive cover, andan integrally molded plastic jacket. The light emitting diode comprisesa contact set comprising a positive contact and a negative contact, eachof the contacts having a first end portion and a second end portion, anda lens body containing a semiconductor chip and the first end portionsof the positive and negative contacts. The electrical wire set comprisesa first electrical wire and a second electrical wire electricallyconnected to the second end portions of the positive contact and thenegative contact, respectively. The light transmissive cover has acavity receiving the lens body, and an opening having at least one ofthe contact set and the electrical wire set passing therethrough. Theintegrally molded plastic jacket is positioned at the opening of thelight transmissive cover to provide a seal at the opening againstmoisture and airborne contaminants.

According to a second aspect of the invention a method is provided formaking a jacketed light emitting diode assembly. The method comprisesproviding a light emitting diode comprising a positive contact and anegative contact of a contact set, each of the contacts having a firstend portion and a second end portion, and a lens body containing asemiconductor chip and the first end portions of the positive andnegative contacts. A first electrical wire and a second electrical wireof an electrical wire set are electrically connected to the second endportions of the positive contact and the negative contact, respectively.The light emitting diode is inserted through an opening and into acavity of a light transmissive cover, so that the contact set and/or theelectrical wire set passes through the opening. A plastic jacket ismolded integrally at the opening of the light transmissive cover toprovide a seal at the opening against moisture and airbornecontaminants.

A third aspect of the invention provides a light string comprising aplurality of light emitting diode assemblies connected to one another,the light emitting diode assemblies comprising a plurality of jacketedlight emitting diode assemblies, comprising a light emitting diode, anelectrical wire set, a light transmissive cover, and an integrallymolded plastic jacket. The contact set comprises a positive contact anda negative contact, each of the contacts having a first end portion anda second end portion, and a lens body containing a semiconductor chipand the first end portions of the positive and negative contacts. Theelectrical wire set comprises a first electrical wire and a secondelectrical wire electrically connected to the second end portions of thepositive contact and the negative contact, respectively. The lighttransmissive cover has a cavity with an opening, the cavity receivingthe lens body, the opening having the contact set and/or the electricalwire set passing therethrough. The integrally molded plastic jacket,which is at the opening of the light transmissive cover, provides a sealat the opening against moisture and airborne contaminants along a lengthof the light string.

In accordance with a fourth embodiment of the invention, a method isprovided for moisture sealing a light-emitting diode elements of a lightstring. The method comprises providing a light string comprising aplurality of light emitting diodes, the plurality of light emittingdiodes comprising a positive contact and a negative contact of a contactset, each of the contacts having a first end portion and a second endportion, and a lens body containing a semiconductor chip and the firstend portions of the positive and negative contacts. First and secondelectrical wires of an electrical wire set are electrically connected tothe second end portions of the positive contact and the negativecontact, respectively. The light emitting diode is inserted through anopening and into a cavity of a light transmissive cover, the openinghaving at least one of the contact set and the electrical wire setpassing therethrough. A plastic jacket is molded integrally at theopening of the light transmissive cover to provide a seal at the openingagainst moisture and airborne contaminants.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are incorporated in and constitute a part ofthe specification. The drawings, together with the general descriptiongiven above and the detailed description of the certain preferredembodiments and methods given below, serve to explain the principles ofthe invention. In such drawings:

FIGS. 1A and 1B show two example block diagrams of the light string inits embodiment preferred primarily, with one diagram for a 110 VACcommon household input electrical source (e.g., 60 Hz) and one diagramfor a 220 VAC common household (e.g., 50 Hz) input electrical source.

FIG. 2A shows a schematic diagrams of an embodiment of this invention inwhich the diodes of the 50 LEDs (series) blocks 102 of FIG. 1 areconnected in the same direction.

FIG. 2B Shows a schematic diagrams of an embodiment of this invention inwhich the diodes of the 50 LEDs (series) blocks 102 of FIG. 1 areconnected in the reverse direction.

FIGS. 3A and 3B show two example block diagrams of the light string inits embodiment preferred alternatively, with one diagram for a 110 VACcommon household input electrical source (e.g., 60 Hz) and one diagramfor a 220 VAC common household (e.g., 50 Hz) input electrical source.

FIG. 4 shows an example schematic diagram of the AC-to-DC power supplycorresponding to the two block diagrams in FIG. 3 for either the 110 VACor the 220 VAC input electrical source.

FIGS. 5A and 5B show example pictorial diagrams of the manufacturedlight string in either its “straight” or “curtain” form (either form maybe manufactured for 110 VAC or 220 VAC input).

FIG. 6 shows an example pictorial diagram of a fiber optic “icicle”attached to an LED and its housing in the light string, where the“icicle” diffuses the LED light in a predetermined manner.

FIG. 7 is a graph of current versus voltage for diodes and resistors.

FIGS. 8A and 8B are a schematic and block diagrams of direct driveembodiments.

FIG. 9 is a plot showing the alternating current time response of adiode.

FIG. 10 is a graph showing measured diode average current response foralternating current and direct current.

FIG. 11 is a graph showing measured AlInGaP LED average and maximum ACcurrent responses.

FIG. 12 is a graph showing measured light output power as a function ofLED current.

FIG. 13 is a graph showing measured GaAlAs LED average and maximum ACcurrent responses.

FIG. 14 shows an unjacketed LED assembly having crimp connectors.

FIG. 15 shows an unjacketed LED assembly having solder connections.

FIG. 16 shows another embodiment of an unjacketed LED assemblycontaining a contact separator.

FIG. 17 shows an unjacketed LED assembly inserted into alight-transmissive cover.

FIG. 18 shows a partially sectioned view of a jacketed LED assemblyaccording to an embodiment of the invention.

FIG. 19 shows a partially sectioned view of a jacketed LED assemblyaccording to another embodiment of the invention.

FIG. 20 shows a non-sectioned view of the jacketed LED assembly of FIG.18 or 19.

FIG. 21 shows a step involved in the manufacture of a jacketed LEDassembly according to another embodiment of the invention.

DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS AND CERTAINPREFERRED METHODS OF THE INVENTION

Reference will now be made in detail to the presently preferredembodiments and methods of the invention as illustrated in theaccompanying drawings, in which like reference characters designate likeor corresponding parts throughout the drawings. It should be noted,however, that the invention in its broader aspects is not limited to thespecific details, representative assemblies and methods, andillustrative examples shown and described in this section in connectionwith the preferred embodiments and methods. The invention according toits various aspects is particularly pointed out and distinctly claimedin the attached claims read in view of this specification, andappropriate equivalents.

It is to be noted that, as used in the specification and the appendedclaims, the singular forms “a,” “an,” and “the” include plural referentsunless the context clearly dictates otherwise.

According to an embodiment of the present invention, a jacketed lightemitting diode assembly is provided, comprising a light emitting diode,an electrical wire set, a light transmissive cover, and an integrallymolded plastic jacket. The light emitting diode comprises a contact setcomprising a positive contact and a negative contact, each of thecontacts having a first end portion and a second end portion, and a lensbody containing a semiconductor chip and the first end portions of thepositive and negative contacts. The electrical wire set comprises afirst electrical wire and a second electrical wire electricallyconnected to the second end portions of the positive contact and thenegative contact, respectively. The light transmissive cover has acavity receiving the lens body, and an opening through which the contactset and/or the electrical wire set passes. The integrally molded plasticjacket is provided at the opening of the light transparent cover toprovide a seal at the opening against moisture and airbornecontaminants. The internal circuitry of the light emitting diodeassembly may include internal resistance elements as known by those ofskill in the art.

An example of a light emitting diode is depicted in FIG. 14 andgenerally designated by reference numeral 1000. The light emitting diode1000 comprises an LED lens (also referred to as a lamp) 1002, andcontacts 1006, 1008. In the illustrated embodiment, the lamp 1002 isdepicted as a dome-shaped member. The lens 1002 may undertake variousother shapes, many of which are known and practiced in the art, e.g.,oblong, cylindrical, pointed, polygonal. The lens 1002 may be made ofvarious materials, such as glass, plastic, or epoxy and may be clear,clear-colored, and/or diffuse-colored. It should be noted that LEDlenses are normally encapsulated in epoxy. Light-emitting elements(e.g., semiconductors, 1003 a in FIGS. 17–19) and internal circuitry(e.g., wire 1003 b in FIGS. 17–19) are housed in the lens 1002. Theconventional LED structure shown in FIGS. 17–19 is for discussionpurposes only. It is to be understood that other structures,arrangements, and configurations suitable for use or known in the LEDart may be used. These elements and circuitry are well known in the artand, therefore, not described herein in detail. It is noted, however,that the internal circuitry may provide for emission of a continuouslight signal, intermittent on-off blinking, and/or intermittent LEDsub-die color changes.

A flanged lens base 1004 is optionally formed at an end of the lens1002, and may form part of a seal of the lens chamber. This lens base1004 can be formed with a “flange” (as illustrated), or without aflange. Protruding through the lens base 1004 are a contact setcomprising a negative contact 1006 and a positive contact 1008 (alsoreferred to in the art as frames or leads) extending parallel to oneanother. Thus, the contacts 1006 and 1008 have first end portions(unnumbered) contained in the lens 1002, and second end portions(unnumbered) outside of the lens 1002. The contacts 1006 and 1008 arepreferably made of a metal or alloy, such as steel, coated steel,copper, or a copper alloy.

The light emitting diode 1000 is assembled to a set of electrical wires1010 and 1012, e.g., drive wires, discussed below. Various mechanicaland chemical mechanisms and means may be employed for attaching thelight emitting diode 1000 to the electrical wires 1010 and 1012. Forexample, FIG. 14 illustrates conventional crimp connectors 1014 and 1016for making the respective connections. Another example is shown in FIG.15, which is substantially identical to the assembly of FIG. 14 butincludes solder connections 1018 and 1020 in place of the crimpconnectors 1014 and 1016, respectively.

As shown in FIG. 16, the light emitting diode assembly may optionallyfurther comprise a contact separator 1022. The contact separator 1022 isplaced between the contacts 1006 and 1008 to prevent accidental contactand resultant shorting thereof. The contact separator 1022 is made of anon-conductive material, such as a plastic. Although FIG. 16 illustratesthe contact separator 1022 used in combination with the crimp connectors1014 and 1016, it is to be understood that other connection means,including, for example, solder, may be used.

The LED assembly further comprises a light transmissive cover 1024. Asshown in FIGS. 17–20, the light transmissive cover 1024 may have agenerally spherical shape with a cylindrical base, although other shapes(e.g., bulb-like, cylindrical, frustum-conical, conical, polygonal,etc.) may be selected. The light-transmissive cover 1024 permits for thefull or partial passage of light emitted from the LED 1000. The lighttransmissive cover 1024 may be made of a transparent material, such asone selected from the group consisting of glass and plastic, such aspolycarbonates. The cover 1024 may be optically clear, tainted colored,frosted, semi-transparent or translucent, and the like for providing thedesired illumination effect. The light-transmissive cover 1024 mayinclude prisms, facets, or other designs or patterns.

The light emitting diode 1000 is inserted through an opening of thelight transmissive cover 1024 base, so that a portion and morepreferably all of the LED lens 1002 is situated in the chamber of thelight transparent cover 1024. Preferably, the opening of the lighttransmissive cover 1024 is sized to be relatively tight yet slidablerelative to the LED lens base 1004. In this manner, the LED 1000 may beretained in the opening of the light transmissive cover, yet permit forinsertion and removal with firm force. Alternatively, a bonding ormechanical securing (e.g., clamping) means may be used to retain thelight emitting diode 1000 relative to the light transmissive cover 1024.

In accordance with embodiments of the present invention, a plasticjacket is integrally molded onto the light emitting diode assembly atthe opening of the light transparent cover to provide a seal at theopening against moisture and airborne contaminants.

An embodiment of a plastic jacket is illustrated in FIGS. 18–20 anddesignated by reference numeral 1030 and 1030A. The jacket 1030, 1030Amay comprise one or more plastic materials, used alone or in combinationwith non-plastics. Preferably but not necessarily, the jacket 1030,1030A consists of one or more plastic materials. Suitable plasticsinclude, for example and not necessarily limitation, polycarbonate (PC),poly(vinyl chloride) (PVC), polypropylene (PP), and any combinationthereof. The plastic material may be optically transparent ornon-transparent, clear or colored.

The plastic jacket 1030, 1030A is integrally molded on the base of thelight transparent cover 1024 to intimately contact electrical wires 1010and 1012. The plastic jacket 1030, 1030A preferably yet optionallycontacts less than the entire surfaces of the light transmissive cover1024, e.g., in FIG. 20 jacket 1030 contacts the base of the cover 1024.The plastic jacket 1030, 1030A may enter into the opening of the lighttransmissive cover 1024, for example, to contact a portion of theinterior of the cover 1024 base. It should be understood that theportion of the light transmissive cover 1024 that the plastic jacket1030, 1030A contacts need not be light transmissive.

In the embodiment illustrated in FIG. 18, the electrical wires 1010 and1012 pass through the opening of the light transmissive cover 1024, andthe plastic jacket 1030 encases (at least) respective regions of theelectrical wire set passing through the opening. Another embodiment isillustrated in FIG. 19, in which the first and second contacts 1006 and1008 pass through the opening of the light transmissive cover 1024. Inthe embodiment illustrated in FIG. 19, the plastic jacket 1030A encasesthe contacts 1006 and 1008, the distal end portions of the electricalwires 1010 and 1012, and the second end portions of the correspondingconnectors 1014 and 1016. (In the event the connectors 1014 and 1016pass through at the opening of the cover 1024, the connectors 1014 and1016 may be considered to be part of the contacts 1006, 1008 or theelectrical wires 1010, 1012.)

Although not shown, the plastic jacket 1030 or 1030A may optionallyencase other components of the LED 1000, including the lamp base 1004, agreater or lesser portion or all of the contacts 1006, 1008, a greateror lesser portion or all of the cover 1024, as well as a greater orlesser portion of the electrical wires 1010 and 1012.

In preferred embodiments of the invention, the plastic jacket 1030 (or1030A) provides a permanent, waterproof (or at least moistureresistant), and corrosion resistant encapsulation for at least thecontacts 1006 and 1008, the connectors 1014 and 1016, and the distalportion of the electrical wires 1010 and 1012. The invention provides anarrangement where a series of LEDs are interconnected in a sealed,waterproof assembly.

As used herein, “integrally molded” jacket refers to a plastic jacketthat has been molded onto, as opposed to pre-molded and subsequentlyapplied to, another member or device, such as a light transmissive cover1024.

A method for making the jacketed LED assembly will now be explained withreference to FIG. 20. Molding is performed with mold die 1050 and acounterpart mold die (not shown). The mold die 1050 includes a cavity1052, an upper opening 1054, and a lower opening 1056. The LED 1000 andthe base of cover 1024 are placed on the mold die 1050 and positioned sothat the end portions of the electrical wires 1010 and 1012, the exposedportions of the contacts 1006 and 1008, the connectors 1014 and 1016,and the contact separator 1022 are disposed in the mold cavity 1052. Thelens 1004 extends through the upper opening 1054 of the mold die 1050 tosituate the lens 1002 outside the mold die 1050. Likewise, the majorityof the light transmissive cover 1024 is situated outside of the mold die1050. The electrical wires 1010 and 1012 extend through the loweropening 1056 of the mold die 1050.

The mold process and techniques will now be described. Verticalinjection molding equipment is preferred as the easiest and mostefficient for machine operators to align the assembled LED lens, contactwire, and light transmissive cover base inside the injection mold cavity(cavities).

Critical to this waterproof, molded process is selecting a moldingtemperature compatible with LED lens encapsulating material. LED lenses1002 are normally formed using an optical grade epoxy. The encapsulatingepoxy properties will vary between manufacturers. Further variations inepoxy material are found when flame retardant compounds are added to theepoxy. At certain temperature thresholds epoxy material will begin tosoften, transitioning back into liquid form. This is known in the art asTG, or Glass Transition temperature. Exceeding the TG temperature of theLED epoxy material during the molding, or jacketing process will softenthe LED lens material, damaging the internal structure of the LED lamp.This is particularly true of this jacketing, or molding process as heatfrom molding is first conducted by the LED contacts (1006 and 1008).This causes softening of the epoxy surrounding the LED “wire bond”(electrical connection inside the LED lamp completing the circuit fromthe LED chip to the wire frame or contacts) and causing disruption ofwire bond contact.

Mold temperature is not of concern when jacketing, or “over-molding”conventional, incandescent lamps as the melting point of the glass“bulb” material is considerably higher than that of the jacketingplastic material.

For jacketing, or over-molding LED lamps one first determines the TGtemperature of the encapsulating epoxy used. Second, the moldtemperature and duration of the injection (jacketing) process areadjusted so the TG temperature is not exceeded. Pre-warming theinjection material (PVC, PP, PS, etc.) in its raw granular, or pelletform will greatly assist material flow, reduce air pockets, or voids inthe finished jackets, as well as reduce molding duration andtemperature.

According to another embodiment of the invention, a light stringcomprising a plurality of LED assemblies is provided, in which at leastone of the LED assemblies comprises a jacketed LED assembly. Morepreferably, a plurality or all of the LED assemblies are jacketed. Thejacketed LED assemblies may have constructions as described above and asillustrated in FIGS. 18 and 19.

In the event that LED assemblies of a light string are to beindividually jacketed, the jacketing process may be conductedsimultaneously on two or more LED assemblies by use of a correspondingnumber of molds, or one or more molds containing multiple cavities forsimultaneous jacketing of multiple LED lamp and contact wire assemblies.Concurrent practice of this molding technique will improve processefficiency.

The jacketed LED assembly of this invention may be used in varioussystems and light strings. Preferred light string systems with which thejacketed LED assembly of aspects of this invention may be used aredescribed in detail below. It should be understood that the followingdescription and attached drawings of preferred devices, apparatuses,assemblies, methods, and the like are exemplary, but not exhaustive asto the scope of environments in which the jacketed LED assemblies andlight strings of the present invention may be used.

The term “alternating current voltage”, sometimes abbreviated as “VAC”,as used herein occasionally refers to a numerical amount of volts, forexample, “220 VAC”. It is to be understood that the stated number ofalternating current volts is the nominal voltage which cyclescontinuously in forward and reverse bias and that the actualinstantaneous voltage at a given point in time can differ from thenominal voltage number.

In accordance with an embodiment of the present invention, an LED lightstring employs a plurality of LEDs wired in series-parallel form,containing at least one series block of multiple LEDs. The series blocksize is determined by the ratio of the standard input voltage (e.g.,either 110 VAC or 220 VAC) to the drive voltage(s) of the LEDs to beemployed (e.g., 2 VAC). Further, multiple series blocks, if employed,are each of the same LED configuration (same number and kinds of LEDs),and are wired together along the string in parallel. LEDs of the lightstring may comprise either a single color LED or an LED includingmultiple sub-dies each of a different color. The LED lenses may be ofany shape, and may be clear, clear-color, or diffuse-color. Moreover,each LED may have internal circuitry to provide for intermittent on-offblinking and/or intermittent LED sub-die color changes. Individual LEDsof the light string may be arranged continuously (using the same color),or periodically (using multiple, alternating CIP colors), orpseudo-randomly (any order of multiple colors). The LED light string mayprovide an electrical interface to couple multiple light stringstogether in parallel, and physically from end to end. Fiber opticbundles or strands may also be coupled to individual LEDs to diffuse LEDlight output in a predetermined manner.

An LED light string of embodiments of the present invention may have thefollowing advantages. The LED light string may last far longer andrequire less power consumption than light strings of incandescent lamps,and the light string may be safer to operate since less heat isgenerated. The LED light string may have reduced cost of manufacture byemploying series-parallel blocks to allow operation directly from astandard household 110 VAC or 220 VAC source, either without anyadditional circuitry (AC drive), or with only minimal circuitry (DCdrive). In addition, the LED light string may allow multiple strings tobe conveniently connected together, using standard 110 VAC or 220 VACplugs and sockets, desirably from end-to-end.

Direct AC drive of LED light string avoids any power conversioncircuitry and additional wires; both of these items add cost to thelight string. The additional wires impose additional mechanicalconstraint and they may also detract aesthetically from the decorativestring. However, direct AC drive results in pulsed lighting. Althoughthis pulsed lighting cannot be seen at typical AC drive frequencies(e.g. 50 or 60 Hz), the pulsing apparently may not be the most efficientuse of each LED device because less overall light is produced than ifthe LEDs were continuously driven using DC. However, this effect may becompensated for by using higher LED current during each pulse, dependingon the pulse duty factor. During “off” times, the LED has time to cool.It is shown that this method can actually result in a higher efficiencythan DC drive, depending on the choice of AC current.

FIG. 1 shows the embodiment of an LED light string in accordance withthe present invention, and as preferred primarily through AC drive. InFIG. 1, the two block diagrams correspond to an exemplary stringemploying 100 LEDs, for either 110 VAC (top diagram) or 220 VAC (bottomdiagram) standard household current input (e.g., 50 or 60 Hz). In thetop block diagram of FIG. 1A, the input electrical interface consistsmerely of a standard 110 VAC household plug 101 attached to a pair ofdrive wires.

With the average LED drive voltage assumed to be approximately 2.2 VACin FIG. 1A, the basic series block size for the top block diagram,corresponding to 110 VAC input, is approximately 50 LEDs. Thus, for the110 VAC version, two series blocks of 50 LEDs 102 are coupled inparallel to the drive wires along the light string. The two drive wiresfor the 110 VAC light string terminate in a standard 110 VAC householdsocket 103 to enable multiple strings to be connected in parallelelectrically from end-to-end.

In the bottom block diagram of FIG. 1B, the input electrical interfacelikewise consists of a standard 220 VAC household plug 104 attached to apair of drive wires. With again the average LED drive voltage assumed tobe approximately 2.2 VAC in FIG. 1B, the basic series block size for thebottom diagram, corresponding to 220 VAC input, is 100 LEDs. Thus, forthe 220 VAC version, only one series block of 100 LEDs 105 is coupled tothe drive wires along the light string. The two drive wires for the 220VAC light string terminate in a standard 220 VAC household socket 106 toenable multiple strings to be connected in parallel from end-to-end.Note that for either the 110 VAC or the 220 VAC light string, thestandard plug and socket employed in the string varies in accordance tothe country in which the light string is intended to be used.

Whenever AC drive is used and two or more series are incorporated in thelight string, the series blocks may each be driven by either thepositive or negative half of the AC voltage cycle. The only requirementof this embodiment is that, in each series block, the LEDs are wiredwith the same polarity; however the series block itself, since driven inparallel with the other series blocks, may be wired in either direction,using either the positive or the negative half of the symmetric ACelectrical power cycle.

FIGS. 2A and 2B show two schematic diagram implementations of the topdiagram of FIG. 1A, where the simplest example of AC drive is shown thatuses two series blocks of 50 LEDs, connected in parallel and powered by110 VAC. In the top schematic diagram of FIG. 2A both of these LEDseries blocks are wired in parallel with the polarity of both blocks inthe same direction (or, equivalently, if both blocks were reversed).With this block alignment, both series blocks flash on simultaneously,using electrical power from the positive (or negative, if both blockswere reversed) portion of the symmetric AC power cycle. A possibleadvantage of this configuration is that, since the LEDs all flash ontogether at the cycle rate (60 Hz for this example), when the lightstring flashes on periodically, it is as bright as possible.

The disadvantage of this configuration is that, since both blocks flashon simultaneously, they both draw power at the same time, and themaximum current draw during this time is as large as possible. However,when each flash occurs, at the cycle rate, the amount of light flashedis maximal. The flash rate, at 50–60 Hz, cannot be seen directly byhuman eye and is instead integrated into a continuous light stream.

The bottom schematic diagram FIG. 2B shows the alternativeimplementation for the top diagram of FIG. 1A, where again, two seriesblocks of 50 LEDS are connected in parallel and powered by 110 VAC.

In this alignment, the two series blocks are reversed, relative to eachother, in polarity with respect to the input AC power. Thus, the twoblocks flash alternatively, with one block flashing on during thenegative portion of each AC cycle. The symmetry, or “sine-wave” natureof AC allows this possibility. The advantage is that, since each blockflashes alternatively, drawing power during opposite phases of the ACpower, the maximum current draw during each flash is only half of thatpreviously (i.e., compared when both blocks flash simultaneously).However, when each flash occurs, at twice the cycle rate here, theamount of light flashed is reduced (i.e., half the light than if twoblocks were flashing at once as previously illustrated). The flash rate,at 100–120 Hz, cannot be seen directly by the human eye and is insteadintegrated into a continuous light stream.

The trade-off between reversing series blocks when two or more exist inan AC driven circuit is influenced primarily by the desire to minimizepeak current draw. A secondary influence has to do with the propertiesof the human eye in integrating periodic light flashes. It is well knownthat the human eye is extremely efficient in integrating light pulsesrapid enough to appear continuous. Therefore, the second form of thelight string is preferred from a power draw standpoint because theeffect on human perception is insignificant.

For AC drive with non-standard input (e.g., three-phase AC) the seriesblocks may similarly be arranged in polarity to divide power among theindividual cycles of the multiple phase AC. This may result in multiplepolarities employed for the LED series blocks, say three polarities foreach of the three positive or negative cycles.

As an alternative preference to AC drive, FIG. 3 shows two blockdiagrams that correspond to an exemplary string employing 100 LEDs andDC drive, for either 110 VAC (top diagram) or 220 VAC (bottom diagram)standard household current input (e.g., 50 or 60 Hz). In the top blockdiagram of FIG. 3A, the input electrical interface consists of astandard 110 VAC household plug 301 attached to a pair of drive wires,followed by an AC-to-DC converter circuit 302. As in FIG. 1, with theaverage LED drive voltage assumed to be approximately 2.2 VAC in FIG.3A, the basic series block size for the top block diagram, correspondingto 110 VAC input, is approximately 50 LEDs. Thus, for the 110 VACversion, two series blocks of 50 LEDs 303 are coupled in parallel to theoutput of the AC-to-DC converter 302 using additional feed wires alongthe light string. The two drive wires for the 110 VAC light stringterminate in a standard 110 VAC household socket 304 to enable multiplestrings to be connected in parallel electrically from end-to-end.

In the bottom block diagram of FIG. 3B, the input electrical interfacelikewise consists of a standard 220 VAC household plug 305 attached to apair of drive wires, followed by an AC-to-DC converter circuit 306. Withagain the average LED drive voltage assumed to be approximately 2.2 VACin FIG. 3B, the basic series block size for the bottom diagram,corresponding to 220 VAC input, is 100 LEDs. Thus, for the 220 VACversion, only one series block of 100 LEDs 307 is coupled to the outputof the AC-to-DC converter 306 using additional feed wires along thelight string. The two drive wires for the 220 VAC light string terminatein a standard 220 VAC household socket 308 to enable multiple strings tobe connected in parallel from end-to-end. Note that for either the 110VAC or the 220 VAC light string, the standard plug and socket employedin the string varies in accordance to the country in which the lightstring is intended to be used.

FIG. 4 shows an example schematic electrical diagram for the AC-to-DCconverter employed in both diagrams of FIG. 3. The AC input to thecircuit in FIG. 1 is indicated by the symbol for an AC source 401. Avaristor 402 or similar fusing device may optionally be used to ensurethat voltage is limited during large power surges. The actual AC to DCrectification is performed by use of a full-wave bridge rectifier 403.This bridge rectifier 403 results in a rippled DC current and thereforeserves as an example circuit only. A different rectification scheme maybe employed, depending on cost considerations. For example, one or morecapacitors or inductors may be added to reduce ripple at only minor costincrease. Because of the many possibilities, and because of theirinsignificance, these and similar additional circuit features have beenpurposely omitted from FIG. 4.

For either the 110 VAC or the 220 VAC version of the LED light string,and whether or not an AC to-DC power converter is used, the finalmanufacturing may be a variation of either the basic “straight” stringform or the basic “curtain” string form, as shown in the top and bottompictorial diagrams in FIGS. 5A and 5B. In the basic “straight” form ofthe light string, the standard (110 VAC or 220 VAC) plug 501 is attachedto the drive wires which provide power to the LEDs 502 via theseries-parallel feeding described previously. The two drive and otherfeed wires 503 are twisted together along the length of the light stringfor compactness and the LEDs 502 in the “straight” form are aligned withthese twisted wires 503, with the LEDs 502 spaced uniformly along thestring length (note drawing is not to scale). The two drive wires in the“straight” form of the light string terminate in the standard(correspondingly, 110 VAC or 220 VAC) socket 504. Typically, the LEDsare spaced uniformly every four inches.

In the basic “curtain” form of the light string, as shown pictorially inthe bottom diagram of FIGS. 5A and 5B, the standard (110 VAC or 220 VAC)plug 501 again is attached to the drive wires which provide power to theLEDs 502 via the series-parallel feeding described previously. The twodrive and other feed wires 503 are again twisted together along thelength of the light string for compactness. However, the feed wires tothe LEDs are now twisted and arranged such that the LEDs are offset fromthe light string axis in small groups (groups of 3 to 5 are shown as anexample). The length of these groups of offset LEDs may remain the samealong the string or they may vary in either a periodic or pseudo-randomfashion.

Within each group of offset LEDs, the LEDs 502 may be spaced uniformlyas shown or they may be spaced nonuniformly, in either a periodic orpseudo-random fashion (note drawing is not to scale). The two drivewires in the “curtain” form of the light string also terminate in astandard (correspondingly 110 VAC or 220 VAC) socket 504. Typically, theLED offset groups are spaced uniformly every six inches along the stringaxis and, within each group, the LEDs are spaced uniformly every fourinches.

In any above version of the preferred embodiment to the LED lightstring, blinking may be obtained using a number of techniques requiringadditional circuitry, or by simply replacing one of the LEDs in eachseries block with a blinking LED. Blinking LEDs are already available onthe market at comparable prices with their continuous counterparts, andthus the light string may be sold with the necessary (e.g., one or two)additional blinkers included in the few extra LEDs.

Typically, the LEDs in the light string will incorporate a lens forwide-angle viewing. However, it is also possible to attach fiber opticbundles or strands to the LEDs to spatially diffuse the LED light in apredetermined way for a desirable visual effect. In such case, the LEDlens is designed to create a narrow-angle light beam (e.g., 20 degreebeamwidth or less) along its axis, to enable the LED light to flowthrough the fiber optics with high coupling efficiency. An example ofthe use of fiber optics is shown in FIG. 6, where a very lossy fiberoptic rod is employed with intention for the fiber optic rod to glowlike an illuminated “icicle.” In FIG. 6, the LED 801 and its housing 802may be attached to the fiber optic rod 803 using a short piece of tubing804 that fits over both the LED lens and the end of the fiber optic rod(note that the drawing is not to scale). An example design uses acylindrical LED lens with a narrow-angle end beam, where the diameter ofthe LED lens and the diameter of the fiber optic rod are the same (e.g.,5 mm or 3/16 inches). The fiber optic rod 803 is typically between threeto eight inches in length and may be either uniform in length throughoutthe light string, or the fiber optic rod length may vary in either aperiodic or pseudo-random fashion.

Although the fiber optic rod 803 in FIG. 6 could be constructed using avariety of plastic or glass materials, it may be preferred that the rodis made in either a rigid form using clear Acrylic plastic or clearcrystal styrene plastic, or in a highly flexible form using highlyplasticized Polyvinyl Chloride (PVC). These plastics are preferred forsafety, durability, light transmittance, and cost reasons. It may bedesirable to add into the plastic rod material either air bubbles orother constituents, such as tiny metallic reflectors, to achieve thedesigned measure of lossiness for off-axis glowing (loss) versus on-axislight conductance. Moreover, it is likely to be desirable to add UVinhibiting chemicals for longer outdoor life, such as a combination ofhindered amine light stabilizer (HALS) chemicals. The tubing 804 thatconnects the fiber optic rod 803 to its LED lens 801 may also made froma variety of materials, and be specified in a variety of ways accordingto opacity, inner diameter, wall thickness, and flexibility. Fromsafety, durability, light transmittance, and cost reasons, it may bepreferred that the connection tubing 804 be a short piece (e.g., 10 mmin length) of standard clear flexible PVC tubing (containing UVinhibiting chemicals) whose diameter is such that the tubing fits snuglyover both the LED lens and the fiber optic rod (e.g., standard walltubing with ¼ inch outer diameter). An adhesive may be used to hold thisassembly more securely.

The method of determining and calculating the preferred LED network thatprovides stable and functioning operation will now be described.

Many current-limiting designs use a single impedance element in seriesbetween the LED network and the power supply. Current-saturatedtransistors are a less common method of current limiting. A resistor isoften used for the impedance element due to low cost, high reliabilityand ease of manufacture from semiconductors. For pulsed-DC or AC power,however, a capacitor or inductor may instead be used for the impedanceelement. With AC power, even though the waveform shape may be changed bycapacitors or inductors, the overall effect of these reactive elementsis basically the same as a resistor, in adding constant impedance to thecircuit due to the single AC frequency involved (e.g., 60 Hz). In anycase, the fundamental effect of current-limiting circuitry is topartially linearize or limit the highly nonlinear current versus voltagecharacteristic response curve of the diode, as shown in FIG. 7 for asingle resistor element.

FIGS. 8A and 8B show the preferred embodiment of the invention, whereina network of diodes, consisting of LEDs, is directly driven by the ACsource without any current-limiting circuitry. FIG. 8A is a generalschematic diagram showing M series blocks of LEDs directly connected inparallel to the AC source where, for the m-th series block, there areN_(m){1≦m≦M} LEDs directly connected to each other in series. Also shownis a reversal of polarity between some series blocks, placing theseblocks in opposite AC phase, in order to minimize peak current in theoverall AC circuit. FIG. 8B is a block diagram of the above schematic,where a combination plug/socket is drawn explicitly to show how multipledevices can be directly connected either on the same end or in anend-to-end fashion, without additional power supply wires in between.This end-to-end connection feature is particularly convenient fordecorative LED light strings.

The invention in FIGS. 8A and 8B may have additional circuitry, notexplicitly drawn, to perform functions other than current-limiting. Forexample, logic circuits may be added to provide various types ofdecorative on-off blinking. A full-wave rectifier may also be used toobtain higher duty factor for the diodes which, without the rectifier,would turn on and off during each AC cycle at an invisibly high rate(e.g., 50 or 60 Hz). The LEDs themselves may be a mixture of any type,including any size, shape, material, color or lens. The only vitalfeature of the diode network is that all diodes are directly driven fromthe AC power source, without any form of current-limiting circuitryexternal to the diode.

In order to directly drive a network of diodes without current-limitingcircuitry, the voltage of each series block of diodes must be matched tothe input source voltage. This voltage matching requirement for directAC drive places fundamental restrictions on the number of diodes on eachdiode series block, depending on the types of diodes used. For thevoltage to be “matched,” in each series block, the peak input voltage,V_(peak), must be less than or equal to the sum of the maximum diodevoltages for each series block. Mathematically, let V_(peak) be the peakvoltage of the input source and let V_(max)(n,m) be the maximum voltagefor the n-th diode {1≦n≦N_(m)} of the m-th series block {1≦m≦M}. Then,for each m, the peak voltage must be less than or equal to the m-thseries block voltage sum,V _(peak)≦Σ_(n) V _(max)(n,m)  (1)where {1≦n≦N_(m)} in the sum over n. For simpler cases where all N_(m)diodes in the m-th series block are of the same type, each with V_(max),then V_(peak)≦N_(m)V_(max).

The maximum voltage V_(max) of each diode is normally defined by thevoltage which produces diode maximum current, I_(max). However, whendiodes of different types are used in a series block, the series blockvalue of I_(max) is the minimum of all individual diode values forI_(max) in the series block. Thus, if the m-th series block has N_(m)diodes, with the n-th diode in the m-th series block having maximumcurrent I_(max)(n,m), then the value of I_(max) for the m-th seriesblock, I_(max)(m), is determined by the minimum of these N_(m)individual diode values,I _(max)(m)=min[I _(max)(n,m); {1≦n≦N _(m)}].  (2)

The maximum voltage V_(max) of each diode in the m-th series block isthus defined as the voltage which produces the m-th series block maximumcurrent I_(max)(m). For simpler cases where all diodes in a series blockare of the same type, each with maximum current I_(max), thenI_(max)(m)=I_(max).

For AC or any other regularly varying input voltage, there is anadditional requirement to direct drive voltage matching. Here, in asimilar way to peak voltage above, the average, or RMS, voltage of thesource, V_(rms), must also be less than or equal to the sum of theaverage diode voltages, V_(avg), for each series block. Mathematically,let V_(rms) be the RMS voltage of the input source and let V_(avg)(n,m)be the average forward voltage for the n-th diode {1≦n≦N_(m)} of them-th series block {1≦m≦M}. Then, for each m, the RMS voltage must beless than or equal to the m-th series block voltage sum,V _(rms)≦Σ_(n) V _(avg)(n,m)  (3)where {1≦n≦N_(m)} in the sum over n. For simpler cases where all N_(m)diodes in the m-th series block are of the same type, each with V_(rms),then V_(rms)≦N_(m)V_(avg).

In a similar way to the peak voltage above, the average voltage of eachdiode, V_(avg) is normally defined by the voltage which produces diodeaverage current, I_(avg). However, when diodes of different types areused in a series block, the series block value of I_(avg) is the minimumof all individual diode values for I_(avg) in the series block. Thus, ifthe m-th series block has N_(m) diodes, each with average currentI_(avg)(n,m) then the value of I_(avg) for the M-th series block,I_(avg)(m), is determined by the minimum of these N_(m) values,I _(avg)(m)=min[I _(avg)(n,m); {1≦n≦N _(m)}].  (4)

The average voltage V_(avg) of each diode in the m-th series block isthus defined as the voltage which produces the m-th series block averagecurrent I_(avg)(m). For simpler cases where all diodes in a series blockare of the same type, each with average current I_(avg), thenI_(avg)(m)=I_(avg).

Note that the term “average”, rather than “RMS,” is used to distinguishRMS diode values from RMS input voltage values because diode values arealways positive (nonnegative) for all positive or negative inputvoltages considered, so that diode RMS values are equal to their simpleaverages. Note also that in past LED designs, the specified DC value forI_(nom) is equated to the average diode value, I_(avg). LEDs are alwaysspecified in DC, and the specified DC value for I_(nom) results from atradeoff between LED brightness and LED longevity. In the direct ACdrive analysis below, this tradeoff between brightness and longevityresults in values for I_(avg) that are generally different than I_(nom).The direct AC drive value for V_(avg) is thus also generally differentthan the LED specified DC value V_(nom).

LEDs are specified in terms of DC values, V_(nom) and I_(nom). For ACpower, since V_(avg) is an AC quantity and V_(nom) is a DC quantity,they are fundamentally different from each other. This basic differencebetween AC and DC values arises from the nonlinear relationship betweendiode voltage and diode current. Consider AC voltage input to a diode asshown for one period in FIG. 9, where the peak voltage shown, V_(pk), isless than or equal to the diode maximum voltage, V_(max). For ACvoltages below the diode voltage threshold, V_(th), the current is zero.As the voltage increases above V_(th) to its peak value, V_(pk), andthen falls back down again, the diode current rises sharply in anonlinear fashion, in accordance to its current versus voltagecharacteristic response curve, to a peak value, I_(pk), and then thediode current falls back down again to zero current in a symmetricfashion. Since the voltage was chosen such that V_(pk)≦V_(max), then thepeak diode current satisfies I_(pk)≦I_(max). The average diode current,I_(avg), is obtained by integrating the area under the current spikeover one full period.

The central problem of AC voltage matching in equations (1) through (4)for direct drive of diodes is to first determine peak AC diode current,I_(peak) and average AC diode current, I_(avg), as a function of V_(rms)or, equivalently, the peak AC voltage V_(peak)=√2 V_(rms). Since thenonlinear relationship for diode current versus voltage is not known inclosed form, these diode AC current versus input AC voltagerelationships cannot be obtained in closed form. Moreover, the nonlineardiode AC current versus input AC voltage relationships vary fordifferent diode types and materials. In all cases, since the diodecurrent versus voltage characteristic curve, near the practicaloperating point V_(nom), is a convex-increasing function, i.e., itsslope is positive and increases with voltage, the average diode currentI_(avg) that results from a given RMS value of AC voltage is alwayshigher than the diode current that would be achieved for a DC voltageinput having the same value. Because of this, specified DC values fordiode voltage cannot be directly substituted for AC diode voltagevalues. Instead, the characteristic diode AC current versus input ACvoltage relationships must be found for the AC waveform of interest.

The characteristic diode AC current versus voltage relationships may befound by measuring diode current values I_(avg) and I_(peak) as afunction of RMS voltage, V_(rms), using variable voltage AC source. Anumber of alike diodes are used in these measurements to obtain goodstatistics. If different diode types or materials are considered, theneach measurement procedure is repeated accordingly. FIG. 10 shows atypical measurement result for average current, I_(avg), where the diodeused has specified nominal values of V_(nom)=2 VDC and I_(nom)=20 mA.

The average AC current curve is always to left of the DC current curvein FIG. 10. Thus, FIG. 10 shows that if one used DC voltages for thediode in an AC circuit, the resulting average AC diode current would bemuch higher than the DC current expected. Recall that in the prior art,where a number of alike 2 VDC LEDs are connected in series with acurrent-limiting resistor, a maximum number N of LEDs is defined bysumming the individual LED voltages and equating to the RMS inputvoltage. For a 120 VAC source, this maximum number is N=60 LEDs. Theprior art then subtracts five or ten LEDs from this maximum to obtain adesign number, and computes the resistor value using the differencebetween the AC input RMS voltage and the sum of these DC LED voltages.This design is marginally stable, and then becomes unstable, as thenumber of LEDs subtracted becomes smaller. Instability is proven in FIG.10, by considering the limit case where a maximum number N=60 of LEDsare used and hence no LEDs are subtracted. In this limit case, one mightargue that a resistor must be used anyway, but according to this designformula, presented for five or ten LEDs subtracted, the resistor valuein this case would equal zero. As FIG. 10 shows, if the resistor valuewere zero, i.e., the resistor is omitted, instead of the DC design valueof I_(nom)=20 mA for LED current (the rightmost, DC, curve at 2.0 VDC),the LED average AC current will be off the scale, higher than themaximum diode current I_(max)=100 mA (the leftmost, AC, curve at 1.87VAC), and the device will fail immediately or almost immediately.

In order to properly perform matching in an direct AC drive design, thecharacteristic diode AC current versus input AC voltage relationshipsmust be measured and used to specify the AC values for equations (1)through (4). DC specifications and DC diode measurements cannot directlybe used in the direct AC drive design, and they are useful only as aguide for theoretical inference, discussed further below. Along with thediode average AC current, the diode peak AC current must also bemeasured as a function of RMS (or equivalently, peak) input AC voltage.FIG. 11 shows a typical measurement result, where the diode used hasspecified DC nominal values of V_(nom)=2 VDC and I_(nom)=20 mA.

As stated previously, for an AC design, the LED average AC current,I_(avg), is generally different from the specified LED nominal DCcurrent, I_(nom). Likewise, the LED maximum AC current, I_(max), is alsogenerally different from the specified LED maximum DC current. Choice ofthese values represent a tradeoff between LED brightness and electricalefficiency versus LED longevity. In general for pulsed-DC or AC input,the LED is off at least part of the time and is therefore has time tocool during off-time while heating during on-time. In order to increaselight output and hence electrical efficiency, both the average and thepeak diode current values can be raised somewhat above specified DCvalues and maintain the same longevity, which is defined as the totalon-time until, say, 30% loss of light output is incurred—typically atabout 100,000 on-time hours. Moreover, these LED average and peakcurrent values can be raised further to increase light output andelectrical efficiency at some expense in LED longevity, depending on theon-time duty factor. Higher ambient temperatures are accounted for bylowering, or “derating” these values somewhat.

In a publication by Hewlett Packard, a number of curves are presented ofprojected long term light output degradation, for various pulsed-DC dutyfactors and various average and peak current values, at ambienttemperature T_(A)=55° C. The AlInGaP LEDs used in this data representsthe material commonly used in an LED with specified DC nominal voltageV_(nom)=2 VDC. While results vary somewhat for other LED materials, itcan be inferred from this data that, for most LEDs specified atI_(nom)=20 mA, the AC design choice for I_(avg) is approximately in theinterval,30 mA≦I_(avg)≦50 mA  (5)where the specific value chosen, I_(avg)=36 mA, is indicated in FIG. 13.

Similarly, from the Hewlett Packard data it can be inferred that, formost LEDs with maximum DC current specified at 100 mA, and the AC designchoice for I_(max) is approximately,I_(max)≦120 mA  (6)where a specific value chosen of I_(max)=95 mA satisfying this, thatcorresponds to V_(avg)=1.6 VAC and I_(avg)=36 mA, is also indicated inFIG. 11.

To clarify the direct AC drive design, consider again the simpler casewhere all N LEDs in a series block are of the same type, with each LEDspecified as before at V_(nom)=2 VDC and I_(nom)=20 mA. Moreover, letthe input AC power be the U.S. standard value and assume V_(rms)=120 VACfor voltage matching. With the above values for I_(max) and I_(avg), themaximum and average LED voltages, V_(max) and V_(avg), are determinedusing AC current versus voltage measurements in FIG. 11 and simplifiedversions of equations (2) and (4), respectively. The minimum number N ofLEDs is determined from these voltages using the input voltageV_(peak)=√2 V_(rms) and equations (1) and (3), for maximum and averagevoltage respectively. Since the value for I_(max)=95 mA was chosen as alower value than possible by equation (6), corresponding to V_(avg)=1.6VAC and I_(avg)=36 mA, the maximum voltage becomes V_(max)=√2 V_(avg)and equation (1) is automatically satisfied by satisfying equation (3).Solving equation (3) results in the minimum number of N LEDs as,V _(rms) ≦NV _(avg)→120≦N(1.6)→N≧75  (7)

Although the value of N=75 is a convenient number to use formanufacturing and sale of a decorative LED light string, if a different,less convenient, minimum number N of LEDs were computed, the result canbe rounded up or down slightly for convenience, provided that thesubsequent changes in LED brightness or longevity are acceptable. Forexample, if the RMS voltage were assumed to be 110 VAC, then theresulting minimum number of LEDs in equation (7) would be N≧69, and thisvalue may be rounded to a final value of N=70 for convenience, with onlyslight impact on LED brightness.

Efficiency of the above direct AC drive design example can be estimatedby first noting that the average power, P_(avg), consumed by a singleLED in the series block is the product of the average voltage and theaverage current, P_(avg)=V_(avg)I_(avg). This is compared against theoptimal DC baseline that uses the specified DC nominal LED powerconsumption, P_(nom), defined as the product of the nominal voltage andthe nominal current, P_(nom)=V_(nom)I_(nom). Using the values given inthe above direct AC drive example, there results, P_(avg)≈1.44 P_(nom),so that the direct AC drive design consumes 44% more power per LED thanthe DC baseline. However, to examine efficiency, first let L_(avg) bethe average light output power for the direct AC drive design and L_(DC)be the optimal light output power using the DC baseline. This lightoutput power L represents LED efficiency as a device, i.e., how muchlight the LED can be made to produce. Defining relative deviceefficiency as the quotient ε_(D)=L_(avg)/L_(DC) enables the amount oflight produced by each LED in direct AC drive design to be compared withthe optimal DC baseline. Using an approximation that the LED lightoutput power, L, is proportional to the LED current, I, this LED deviceefficiency, ED, is approximately,ε_(D) =L _(avg) /L _(DC) ≈I _(avg) /I _(nom)=36/20=1.8  (8)so that the direct AC design example makes about 80% more use of eachLED as a light producing device than the optimal DC baseline. In otherwords, for each LED used, the direct AC drive design produces about 80%more light than the maximum possible by a DC design based on nominal LEDvalues. Although this factor of 80% light increase appears to be large,its effect is diminished by human perception. According to the wellknown law by Stevens, human perceptions follow a continuum given by thepower relationship,B∝L^(ρ)  (9)where L is the stimulus power, B is the perceived brightness intensity,and exponent ρ is a parameter that depends on the type of stimulus. Forlight stimuli, L is the light power in Watts, B is the perceivedphotopic brightness in lumens, and the exponent is approximately ρ≈⅓.With this exponent, the 80% increase in light output power offered bythe direct AC design example translates into about 22% increase inperceived brightness. Although a smaller realized effect, the direct ACdesign example does offer an increase, rather than a decrease, inbrightness relative to the optimal DC baseline.

LED electrical efficiency, E, is defined by dividing light output powerby electrical power used, E=L/P. Defining relative electrical efficiencyas the quotient ε_(E)=E_(avg)/E_(DC) enables the electrical efficiencyin direct AC drive design to be compared with the optimal DC baseline.Using again an approximation that the LED light output power, L, isproportional to the LED current, I, there follows,ε_(E)≈(I _(avg) /P _(avg))/(I _(nom) /P _(nom))=V _(nom) /V_(avg)=2.0/1.6=1.25  (10)so that the AC direct drive design is about 25% more electricallyefficient than the optimal DC baseline. In other words, for a fixedamount of input power, the direct AC design examples produces about 25%more light than the maximum possible by DC based on nominal LED values.

There are two basic reasons for the results in equations (8) and (10).First, the direct drive design does not have current-limiting circuitryto consume power. If this were the only factor involved, the direct ACdesign efficiency would be 100%, relative to the optimal DC baseline,because the optimal DC baseline is computed without current-limitingcircuitry loss. The second basic reason stems from the nonlinearrelationship between LED current and voltage. Because this relationshipis a convex-increasing function, i.e., its slope is positive andincreases with voltage, average AC diode current I_(avg) is alwayshigher than DC current for the same voltage value. This higher ACaverage current in turn leads to higher average light output, with anapproximation showing a proportional relationship. This is a fundamentaladvantage to the pulsed waveforms over DC that others fail to recognizefor AC and instead try to avoid. The nonlinear current versus voltagerelationship is further taken advantage of in the direct AC drive designby increasing the average current to a more optimal value, using thefact that the LED has time to cool during the off-time interval in eachAC cycle.

An approximation that LED light output is proportional to LED current isvery close for most operating values of LED current, but theapproximation usually overestimates light output at high current values.A typical curve for AlInGaP LEDs, the common material type for LEDs witha 2 VDC specification, is shown in FIG. 12. With this measured result,the relative direct AC drive efficiencies computed in equations (8) and(10) are lowered somewhat, but they are still well above unity. Anumerical integration using FIG. 12 indicates that equations (8) and(10) overestimate efficiency of the direct AC design in the examplepresented by about 15%, and closer estimates for the above relativeefficiencies are ε_(D)≈1.53 and ε_(E)≈1.06.

Diminishing light output power at high LED current places the optimalvalue for RMS and peak LED current values, I_(avg) and I_(max), at aslightly lower value than the average and peak current constraints inequations (5) and (6) allow. For example, FIG. 11 shows that the largestvalue allowed by equations (5) and (6) for V_(avg) is 1.65 VAC, ratherthan the value of 1.60 VAC used above. This larger value of V_(avg)=1.65VAC, achieved by N=72 LEDs in a 120 VAC series block, is slightly lessefficient, as well as slightly less reliable, than the value ofV_(avg)=1.60 VAC and N=75 LEDs. However, the value of N=72 LEDs in theseries block would cost less to produce per unit. Using 110 VAC insteadof 120 VAC to obtain a lower number N=69 LEDs in the series block yieldsyet slightly lower efficiency and reliability still. For decorative LEDlight strings, this final direct AC drive tradeoff between, say, 70versus 75 LEDs in the series block exemplified is a matter of practicaljudgment to provide the highest quality product at the lowest unit cost.

Although it has been shown above that LED specified DC values cannot bedirectly used in for direct AC drive, these values do have sometheoretical utility for using a smaller measurement set to estimate theAC design values. The theoretical basis of this estimation procedureresults from applying statistical inference on the LED specifications,using these specifications in a different way than they are obtained orintended.

LEDs are specified by two voltage parameters, a typical, or “nominal”voltage, V_(nom), and a largest, or “supremum” (usually called “maximum”by LED manufacturers) voltage, V_(sup). These specifications areobtained as ensemble estimates, for a large ensemble of alike LEDs, of“typical” and “largest” DC voltages to expect, from variations due tomanufacturing, that produce the chosen nominal value of DC current,I_(nom). The nominal DC voltage, V_(nom), is intended as a “typical”value for the LED, obtained either by averaging measurements or bytaking the most likely, or modal, value in a measurement histogram. Themaximal DC voltage, V_(sup), is intended as a largest, or “supremum,”value for the LED, obtained by sorting the largest voltage valuemeasured that produces the chosen nominal value of DC current, I_(nom).

The theoretical problem of interest is to obtain values for average ACvoltage, V_(avg), and maximum AC voltage, V_(max), that produce averageAC current, I_(avg) and maximum AC current, I_(max), respectively. Thesevoltage values V_(avg) and V_(max) do not consider LED ensemblevariations due to manufacturing but instead rely on a large enoughnumber N of LEDs in each AC series block for manufacturing variations tobe averaged over. Otherwise, voltage equations (1) and (3) above must bealtered slightly to account for expected LED manufacturing variations.Such an alteration would rely on a statistical model obtained bymeasuring variations of the characteristic AC current versus AC voltagecurve, from LED to LED in a large ensemble of alike LEDs. In any event,the voltages V_(avg) and V_(max) are fundamentally defined to representcharacteristic estimates of voltage for varying values of LED current,obtained by averaging over the ensemble, rather than ensemble estimates,using individual LEDs within the ensemble, of voltages that produce afixed, say, nominal, value of LED current.

In order to make theoretical inferences from LED specifications, it mustbe assumed that the specified ensemble random variables representing“nominal” and “supremum” voltages can be interchanged with equivalentcharacteristic random variables representing corresponding voltages thatproduce corresponding LED current over time. This assumption is similarto a commonly assumed form of ergodicity in random process theory thatinterchanges ensemble random variables with corresponding time-seriesrandom variables.

With this ergodicity assumption, the AC average and maximum voltagevalues of interest, V_(avg) and V_(max), can be inferred from thespecified diode values for DC nominal and maximum voltage, V_(nom) andV_(sup), respectively, using appropriate DC-to-AC scaling between them.It is desired to obtain a single scale factor a for all LED materials,colors, and LED manufacturers. In trying to find this single value forscale factor α, difficulty arises in that the specified voltages,V_(nom) and V_(sup), are fundamentally different for different LEDdopant materials. However, given a specific LED dopant material “M”,such as AlInGaP or GaAlAs, the variations in V_(nom) and V_(sup) acrossapplicable colors and manufacturers are small enough to be consideredfairly insignificant.

Recall that V_(max) is equated with peak input voltage V_(peak) inequation (1), and V_(avg) is equated with RMS input voltage V_(rms) inequation (3). For AC power, the quotient V_(peak)/V_(rms)=√2. It wouldthus be desirable if the quotient V_(sup)/V_(nom) were also always aconstant, preferably equal to √2, so that a single scale factor α_(M)could be used for each LED material, “M.” Unfortunately, this ratio alsovaries significantly for different LED materials. As a result, twodistinct scale factors α_(M) and β_(M) are required for each LEDmaterial composition, “M.” With these material-dependent scale factors,α_(M) and β_(M), the AC voltages of interests are estimated from DCspecified values using,V _(avg)≈α_(M) V _(nom) , V _(max)≈β_(M) V _(sup).  (11)where the scale factors α_(M) and β_(M) are determined by measurement.The advantage provided by this theoretical estimation procedure is thatthe set of measurements determining characteristic curves for peak andaverage AC current versus AC voltage need only be obtained for each LEDdopant material, independent of LED color and LED manufacturer. Ofcourse, the disadvantage to this procedure is that it is approximatewhen compared to making full measurement sets for all specific types ofLEDs considered, and hence some experimentation with the exact number ofLEDs is required.

For AlInGaP LEDs, V_(nom)=2.0 VDC and V_(sup)=2.4 VDC represent thecentroids of specified values across applicable colors and from variousmanufacturers. The characteristic curves presented in FIG. 7 wereobtained from AlInGaP LEDs. From FIG. 11, and the criteria for averageand maximum AC current defined in equations (5) and (6), respectively,AC current values I_(avg)=36 mA and I_(max)=95 mA were chosenpreviously, with V_(max)=√2 V_(avg) and V_(avg)=1.6 VAC. Equations (11),then, lead to α_(AlInGaP)=0.80 and β_(AlInGaP)=0.94. These values may beused theoretically in equations (11) to estimate approximate AC averageand maximum voltages, V_(avg) and V_(max), for other AlInGaP LEDs.

FIG. 13 shows measured characteristic curves for a different set ofalike LEDs, where the dopant material is GaAlAs, rather than AlInGaP.For GaAlAs LEDs, V_(nom)=1.7 VDC and V_(sup)=2.2 VDC represent thecentroids of specified values across applicable colors and from variousmanufacturers. From FIG. 13, and the criteria for average and maximum ACcurrent defined in equations (5) and (6), respectively, AC currentvalues I_(avg)=38 mA and I_(max)=95 mA are chosen, with again V_(max)=√2V_(avg), but now V_(avg)=1.45 VAC. Equations (11), then, lead toα_(GaAlAs)=0.85 and β_(GaAlAs)=0.93. These values may be usedtheoretically in equations (11) to estimate approximate AC average andmaximum voltages, V_(avg) and V_(max), for other GaAlAs LEDs. Note that,with 120 VAC assumed for the RMS input voltage, this value V_(avg)=1.45VAC leads to N=83 LEDs per series block. Similarly, with 110 VAC assumedfor the RMS input voltage, N=76 LEDs per series block. Rounding thesevalues leads to either 75, 80, or 85 LEDs per series block in amanufactured product, with N=75 being most desirable for a decorativeLED light string from a cost basis, if it is sufficiently reliable.

The above direct AC drive design procedure has been verified by buildingnumerous decorative LED light string prototypes using a variety ofdopant materials, colors, and manufacturers. Many of these prototypeswere built as long as two years ago, and all prototypes have remainedoperating continuously without any sign of impending failure. Moreover,a number of these prototypes were subjected to harsh voltage surge andvoltage spike conditions. Voltage surge conditions were produced usinghigh power appliances in the same circuit, all of which failed toproduce anything other than at most some flickering. In about half ofthese experiments the voltage surges created caused circuit breakers totrip.

Voltage spikes, simulating lightning discharges, were produced byinjecting 1000 V, 10 A pulses of up to 10 ms duration and one secondapart into a 100 A main circuit of a small home using a pulse generatorand 10 kW power amplifier. The amplifier was powered from the mainelectrical input of an adjacent home. During these tests, all decorativeLED light string prototypes merely flickered in periodic succession atone second intervals. In the meantime during these tests, the protectivecircuitry of adjoining electronic equipment shut off without any ensuingdamage. All these tests verified conclusively that the decorative LEDlight strings were designed to be highly reliable by the direct AC drivemethod, without the use of any current-limit circuitry.

It is to be understood, however, that current-limit circuitry may beomitted in accordance with embodiments of the invention, omission of thecurrent-limit circuitry is not required. Instead, current-limitcircuitry, such as a resistor, may be included in the circuit as anindependent element or as part of an LED. For example, one or more LEDin a series of LEDs may be equipped with a known drop-down resistorintegrally formed as part of the LED.

It will be understood that various changes in the details, materials andarrangements of the parts which have been described and illustrated inorder to explain the nature of this invention may be made by thoseskilled in the art without departing from the principle and scope of theinvention as expressed in the following claims.

This application incorporates the following disclosures by reference:U.S. Pat. No. 6,461,019, U.S. Pat. No. 6,072,280; application Ser. No.09/378,631 filed Aug. 20, 1999 titled Preferred Embodiment to Led LightString, now abandoned; and application Ser. No. 09/339,616 filed Jun.24, 1999, titled Preferred Embodiment to Led Light String.

The foregoing detailed description of the preferred embodiments of theinvention has been provided for the purpose of explaining the principlesof the invention and its practical application, thereby enabling othersskilled in the art to understand the invention for various embodimentsand with various modifications as are suited to the particular usecontemplated. This description is not intended to be exhaustive or tolimit the invention to the precise embodiments disclosed. Modificationsand equivalents will be apparent to practitioners skilled in this artand are encompassed within the spirit and scope of the appended claims.

1. A jacketed light emitting diode assembly, comprising: a lightemitting diode comprising a contact set comprising a positive contactand a negative contact, each of the contacts having a first end portionand a second end portion; and a lens body containing a semiconductorchip and the first end portions of the positive and negative contacts;an electrical wire set comprising a first electrical wire and a secondelectrical wire electrically connected to the second end portions of thepositive contact and the negative contact, respectively; a lighttransmissive cover having a cavity with an opening, the cavity receivingthe lens body, the opening having at least one of the contact set andthe electrical wire set passing therethrough; and an integrally moldedplastic jacket at the opening of the light transmissive cover to providea seal at the opening against moisture and airborne contaminants.
 2. Ajacketed light emitting diode assembly according to claim 1, whereinelectrically connected portions of the contact set and the electricalwire set are encased by the integrally molded plastic jacket to providea seal against moisture and airborne contaminants.
 3. A jacketed lightemitting diode assembly according to claim 1, wherein the electricalwire set passes through the opening, and wherein the integrally moldedplastic jacket encases respective regions of the electrical wire setpassing through the opening.
 4. A jacketed light emitting diode assemblyaccording to claim 1, wherein the contact set passes through theopening, and wherein the integrally molded plastic jacket encaseselectrical connections between the contact set and the electrical wireset.
 5. A jacketed light emitting diode assembly according to claim 1,wherein the integrally molded plastic jacket comprises at least one ofpolycarbonate, polystyrene, and other moldable plastic material.
 6. Ajacketed light emitting diode assembly according to claim 1, wherein theintegrally molded plastic jacket comprises a plastic selected frompoly(vinyl chloride).
 7. A jacketed light emitting diode assemblyaccording to claim 1, wherein the integrally molded plastic jacketcomprises polypropylene.
 8. A jacketed light emitting diode assemblyaccording to claim 1, wherein the light transmissive cover is at leastpartially encased by the integrally molded plastic jacket.
 9. A methodfor making a jacketed light emitting diode assembly, comprising:providing a light emitting diode comprising a positive contact and anegative contact of a contact set, each of the contacts having a firstend portion and a second end portion, and a lens body containing asemiconductor chip and the first end portions of the positive andnegative contacts; electrically connecting a first electrical wire and asecond electrical wire of an electrical wire set to the second endportions of the positive contact and the negative contact, respectively;inserting the light emitting diode through an opening and into a cavityof a light transmissive cover, the opening having at least one of thecontact set and the electrical wire set passing therethrough; andmolding a plastic jacket integrally at the opening of the lighttransmissive cover to provide a seal at the opening against moisture andairborne contaminants.
 10. A method according to claim 9, whereinelectrically connected portions of the contact set and the electricalwire set are encased by the integrally molded plastic jacket to providea seal against moisture and airborne contaminants.
 11. A methodaccording to claim 9, wherein the electrical wire set passes through theopening, and wherein the plastic jacket encases respective regions ofthe electrical wire set passing through the opening.
 12. A methodaccording to claim 9, wherein the contact set passes through theopening, and wherein the plastic jacket encases electrical connectionsbetween the contact set and the electrical wire set.
 13. A methodaccording to claim 9, wherein the plastic jacket comprises at least oneof polycarbonate, polystyrene, or other moldable plastic material.
 14. Amethod according to claim 9, wherein the plastic jacket comprises aplastic selected from poly(vinyl chloride).
 15. A method according toclaim 9, wherein the plastic jacket comprises polypropylene.
 16. Amethod according to claim 9, wherein the light transmissive cover is atleast partially encased by the integrally molded plastic jacket.
 17. Amethod according to claim 9, wherein said molding comprises injectionmolding the plastic jacket in a mold cavity.
 18. A light stringcomprising a plurality of light emitting diode assemblies connected toone another, the light emitting diode assemblies comprising a pluralityof jacketed light emitting diode assemblies, comprising: a lightemitting diode comprising a contact set comprising a positive contactand a negative contact, each of the contacts having a first end portionand a second end portion; and a lens body containing a semiconductorchip and the first end portions of the positive and negative contacts;an electrical wire set comprising a first electrical wire and a secondelectrical wire electrically connected to the second end portions of thepositive contact and the negative contact, respectively; a lighttransmissive cover having a cavity with an opening, the cavity receivingthe lens body, the opening having at least one of the contact set andthe electrical wire set passing therethrough; and an integrally moldedplastic jacket at the opening of the light transmissive cover to providea seal at the opening against moisture and airborne contaminants along alength of the light string.
 19. A light string according to claim 18,wherein electrically connected portions of the contact set and theelectrical wire set are encased by the integrally molded plastic jacketto provide a seal against moisture and airborne contaminants.
 20. Alight string according to claim 18, wherein the electrical wire setpasses through the opening, and wherein the integrally molded plasticjacket encases respective regions of the electrical wire set passingthrough the opening.
 21. A light string according to claim 18, whereinthe contact set passes through the opening, and wherein the integrallymolded plastic jacket encases electrical connections between the contactset and the electrical wire set.
 22. A light string according to claim18, wherein the integrally molded plastic jacket comprises at least oneof polycarbonate, polystyrene, or other moldable plastic material.
 23. Alight string according to claim 18, wherein the integrally moldedplastic jacket comprises a plastic selected from poly(vinyl chloride).24. A light string according to claim 18, wherein the integrally moldedplastic jacket comprises polypropylene.
 25. A light string according toclaim 18, wherein the light transmissive cover is at least partiallyencased by the integrally molded plastic jacket.
 26. A light stringaccording to claim 18, the light emitting diode assemblies being adaptedto be directly connected to an AC power source and providing a stablelight source, the light string formed by a method of determining aworkable number of light emitting diodes directly coupled in series tothe AC power source to provide the stable light source, said methodcomprising the following steps: determining an RMS voltage rating(V_(RMS)) of said AC power source; determining an AC voltage rating fora particular type of light emitting diodes by determining an averagevoltage drop (V_(avg)) across said particular type of light emittingdiode when connected to said AC power source; calculating said workablenumber (WN) of said light emitting diodes according to the followingformulaWN=V _(RMS) /V _(avg); and electrically interconnecting a plurality ofsaid particular type of light emitting diodes equal to said workablenumber (WN) in series directly to said AC power source without anyintermediate circuit altering devices.
 27. A method for moisture sealinga light-emitting diode elements of a light string, comprising: providinga light string comprising a plurality of light emitting diodes, theplurality of light emitting diodes comprising a positive contact and anegative contact of a contact set, each of the contacts having a firstend portion and a second end portion, and a lens body containing asemiconductor chip and the first end portions of the positive andnegative contacts; electrically connecting a first electrical wire and asecond electrical wire of an electrical wire set to the second endportions of the positive contact and the negative contact, respectively;inserting the light emitting diode through an opening and into a cavityof a light transmissive cover, the opening having at least one of thecontact set and the electrical wire set passing therethrough; andmolding a plastic jacket integrally at the opening of the lighttransmissive cover to provide a seal at the opening against moisture andairborne contaminants.
 28. A method according to claim 27, whereinelectrically connected portions of the contact set and the electricalwire set are encased by the integrally molded plastic jacket to providea seal against moisture and airborne contaminants.
 29. A methodaccording to claim 27, wherein the plastic jacket comprises at least oneof polycarbonate, polystyrene, or other moldable plastic material.
 30. Amethod according to claim 27, wherein the plastic jacket comprises aplastic selected from poly(vinyl chloride).
 31. A method according toclaim 27, wherein the plastic jacket comprises polypropylene.
 32. Amethod according to claim 27, wherein the light transmissive cover is atleast partially encased by the integrally molded plastic jacket.
 33. Amethod according to claim 27, wherein said molding comprises injectionmolding the plastic jacket in a mold cavity.
 34. A method according toclaim 27, further comprising determining a workable number of the lightemitting diode assemblies directly coupled in series to an AC powersource to provide a stable light source, said determining of a workablenumber comprising the following steps: determining an RMS voltage rating(V_(RMS)) of said AC power source; determining an AC voltage rating fora particular type of light emitting diodes by determining an averagevoltage drop (V_(avg)) across said particular type of light emittingdiode when connected to said AC power source; calculating said workablenumber (WN) of said light emitting diodes according to the followingformulaWN=V _(RMS) /V _(avg); and electrically interconnecting a plurality ofsaid particular type of light emitting diodes equal to said workablenumber (WN) in series directly to said AC power source without anyintermediate circuit altering devices.